1. Field of the Invention
The present invention relates to a lens actuator including an objective lens for condensing a light beam on an optical storage medium and driving the objective lens in at least a focusing direction and either a tracking direction or a radial direction relative to the optical storage medium, an optical pickup devise in which the lens actuator is installed, and an optical recording and reproducing apparatus.
2. Description of the Related Art
Some lens actuators of optical pickup devices include a diffractive element for providing compatibility with optical storage media of plural types of standards relative to light sources of plural different wavelengths.
FIG. 1A is a cross-sectional view showing an example of a conventional lens actuator and FIG. 1B is a perspective view showing the conventional lens actuator in FIG. 1A. In FIGS. 1A and 1B, an objective lens 52 and a diffractive element 53 are directly installed on a lens holder 51. Specifically, a passage hole 54 having an effective diameter of a light beam in an optical system is formed at a center portion of the lens holder 51 used as a box casing on which a coil for generating driving power and an elastic support structure not shown in the drawings are installed. The objective lens 52 and the diffractive element 53 are fixed on upper and lower ends of the passage hole 54 so as to have the same optical axis.
FIG. 2A is a cross-sectional view showing an example of a lens actuator on which the diffractive element is disposed so as to have a tilted optical axis relative to the optical axis of the objective lens. FIG. 2B is a perspective view showing the lens actuator in FIG. 2A. In a lens unit in which the objective lens 52 and the diffractive element 53 are integrated, the diffractive element 53 is disposed on the lens holder 51 in a tilted manner relative to the optical axis of the objective lens 52 so as to prevent generation of flare from the diffractive element 53 (refer to Patent Document 1)
Concerning lens holders having the above-mentioned structure, there have been techniques for installing and fixing the objective lens or the diffractive element used as optical elements. For example, as shown in Patent Document 2, a shape of an installed portion of the objective lens is configured so as to prevent influx of adhesive.                Patent Document 1: Japanese Laid-Open Patent Application, No. 2006-139874        Patent Document 2: Japanese Laid-Open Patent Application No. 8-315393        
The invention disclosed in Patent Document 2 is capable of preventing the influx of adhesive. However, a thickness of the holder is increased in an entire circumference of the lens holder as long as a height for fitting into a cylindrical surface of an outside diameter of a lens. Lightning and maintaining when plural optical elements are installed are not considered.
Patent Document 1 discloses a structure of a lens unit in which the objective lens and the diffractive element are adjusted and fixed on the lens holder (support member) and the diffractive element is fixed in a tilted manner. However, Patent Document 1 does not particularly disclose a method for installing the diffractive element in a tilted manner with high accuracy.
In general, an outer shape of the optical element used as a rotator rotating on the optical axis is formed into a cylindrical shape or columnar shape so as to facilitate manufacturing and improve installation accuracy. Further, a round hole or a stepped round hole is formed in a member on which the optical element is installed and positioning is performed through fitting between the hole and a shaft.
Upon installing such an optical element on a movable casing such as the lens holder, there has been a problem of having both readiness of manufacturing the casing and positional accuracy of the optical element in that the optical element and the casing per se used as a positioning subject form a movable unit. In the movable unit requiring high-speed and high-accuracy following capability such as the lens holder, a center of gravity must correspond to a driving center or a center of an elastic support member movably supporting the casing so as to have a small and light movable unit and prevent undesired accompanied movement.
In terms of movable functions, preferably, a shape or a structure for positioning and fixing the optical element is minimized. Further, in a case of a positioning support structure by contact among parts, namely, a three-point supporting structure, for example, it is desirable to minimize an amount of contact and have a large relative position and distance among three points taking into consideration unevenness, margin or error, or surface roughness of the parts.
Although the optical element per se is configured to be light and small, the optical element is required to be enlarged in accordance with a shape structure necessary for positioning relative to the effective diameter in the optical element. The shape structure necessary for positioning is also required in the casing used as the movable unit and is required to have no additional size relative to a driving power generating structure and an elastic support structure disposed on an outer shape portion.
Further, in a case of the general axis of the columnar shape or the stepped shape for positioning using a hole, problems of a rounded portion formed at an edge of a corner on the axis side or a small curved surface portion formed at an edge of a corner on the hole side are likely to hinder having a minimized and simple structure.
In accordance with the above-mentioned facts, in the lens actuator, it is difficult to propose an effective relative positioning structure for the optical element due to optical functions, mechanical functions, or required functions and limitations in terms of manufacturing thereof.
In addition to the above-mentioned description, some optical pickup devices include a diffractive element for providing compatibility with optical storage media of plural types of standards relative to light sources of plural different wavelengths. For example, Patent Document 1 discloses a lens unit in which an objective lens and the diffractive element are integrated. In the disclosed structure, the diffractive element is disposed on a support member in a tilted manner relative to the optical axis of the objective lens so as to prevent generation of flare from the diffractive element.                Patent Document 1: Japanese Laid-Open Patent Application No. 2006-139874        
Patent Document 1 discloses a structure of a lens unit in which the objective lens and the diffractive element are adjusted and fixed on the support member and the diffractive element is fixed in a tilted manner. However, Patent Document 1 does not particularly disclose a method for installing the diffractive element in a tilted manner with high accuracy.
In general, an outer shape of the optical element used as a rotator rotating on the optical axis is formed into a cylindrical shape or columnar shape so as to facilitate manufacturing and improve installation accuracy. Further, a round hole or a stepped round hole is formed in a member on which the optical element is installed and positioning is performed through fitting between the hole and a shaft.
Further, when the diffractive element having a concentric structure is used as en element compatible with plural laser wavelengths, the following conditions (1) and (2) are required so as to sufficiently provide functions thereof:
(1) the optical axis of the objective lens and an optical axis of the diffractive element are positioned with high accuracy, and
(2) both surfaces of the compatible diffractive element have a stepped shape and an end surface of each stepped shape has a plane surface, so that an integrated amount of plane surfaces is equivalent to a plane surface of an entire effective diameter and the diffractive element is disposed in a tilted manner relative to the optical axis of the objective lens so as to prevent flare from regular reflection.
In this case, a center of the diffractive element to be positioned with high accuracy relative to the optical axis of the objective lens corresponds to a center of one of both surfaces of the diffractive element facing the objective lens.
In order to position the center of the tilted surface with high accuracy as in the above-mentioned diffractive element, it is necessary to dispose a holding hole tilted relative to an objective lens holding hole in a casing. However, when such a shape is manufactured using a resin forming mold, it is impossible to manufacture the shape using a normal metal mold structure. A movable mold having a partial tilt is hardly capable of forming a shape with high accuracy for parts requiring reduction in size and weight as in this lens unit. In other words, such parts cannot be manufactured or even if the parts are manufactured, it is impossible to obtain accuracy required for installation.
When members having such a shape are used, there is an increased possibility of occurrence of problems in basic functions and reliability.
Patent Document 1 discloses a structure in which the objective lens and the diffractive element are installed on the support member. However, by using such a dedicated support member, a size of the lens unit as a movable casing may be enlarged and mass of a balancing weight for the lens unit may be increased. In accordance with this, sensitivity or a high resonance frequency is likely to be reduced due to the enlarged size and the increased balancing weight.
A lens actuator must control an objective lens at high speed for it to have a desired position by electrical direct modulation. In order to achieve this, it is required that a movable unit including a lens holder, for example, holding the objective lens be light, a structure of the movable unit be highly rigid so as not to experience internal deformation, and the structure and components be resistant to heat generated from electrical power consumption by a drive coil for driving the movable unit in a predetermined direction.
Further, it is necessary to satisfy the Abbe principle as a principle required for a structure of a precision drive mechanism. An idea of the Abbe principle is that driving force needs to act on a center of a support spring relative to low-frequency driving characteristics and the driving force needs to act on a center of inertia relative to high-frequency driving characteristics so as not to cause tilt when translation is desired or so as not to cause a reverse result. In other words, in a case of the lens actuator, it is required that a thrust center of the drive coil for driving the movable unit in a predetermined direction, an elastic center of the support spring supporting the movable unit, and the center of inertia of mass distribution correspond to one another.
An objective lens having a high aperture ratio for high density mass storage used in a pickup actuator for plural types of laser wavelengths has larger mass compared with a conventional objective lens. In accordance with this, weight for having the corresponding center of inertia is increased so as to satisfy the Abbe principle. Further, addition of a diffractive element compatible with the plural types of laser wavelengths results in an increase of the mass. Such an increase of the mass in the movable unit reduces sensitivity in an inertial field (high-frequency field), so that electrical power consumption is likely to be increased.
The increase of electrical power consumption is likely to cause a heat-related problem for parts due to an increase of an amount of heat generation. Further, if a part having relatively large mass is installed on an outer portion, a ratio of rigidity/mass distribution of a movable unit structure is reduced and resonance characteristics are reduced due to internal deformation. Accordingly, control characteristics are likely to be reduced.
Further, Patent Document 1 discloses a lens unit in which the objective lens and the diffractive element are integrated. In the disclosed structure, the diffractive element is disposed on a support member in a tilted manner relative to an optical axis of the objective lens so as to prevent generation of flare from the diffractive element.
While the lens unit disclosed in Patent Document 1 has merits in that the lens unit may be handled in the same manner as a conventional objective lens unit, a portion of the movable unit is a size larger due to an increase of an outside diameter by the support member relative to the outer portion of the objective lens. This would cause at least one of mass increase and reduced rigidity. Further, in accordance with the relationship with the aperture ratio of the objective lens for an optical disk, the objective lens must be disposed at an upper end of the movable unit. Accordingly, all of the objective lens, the diffractive element, and the support member are positioned on an upper side of the movable unit and the mass increase of an inertia weight for this structure becomes greater. Further, because a positional relationship is established such that the diffractive element is embedded in the support member, a distance to the drive coil as a heat source is reduced. In accordance with this, reliability of optical characteristics is likely to be reduced unless a structure for preventing heat transfer from the drive coil or means for accelerating heat radiation is employed.                Patent Document 1: Japanese Laid-Open Patent Application No. 2006-139874        
In general, an outer shape of an optical element used as a rotator rotating on an optical axis is formed into a cylindrical shape or columnar shape so as to facilitate manufacturing and improve installation accuracy. Further, a round hole or a stepped round hole is formed in a member on which the optical element is installed and positioning is performed through fitting between the hole and a shaft.
Further, when the diffractive element having a concentric structure is used as en element compatible with plural laser wavelengths, the following conditions (1) and (2) are required so as to sufficiently provide functions thereof:
the optical axis of the objective lens and an optical axis of the diffractive element are positioned with high accuracy, and
both surfaces of the compatible diffractive element have a stepped shape and an end surface of each stepped shape has a plane surface, so that an integrated amount of plane surfaces is equivalent to a plane surface of an entire effective diameter and the diffractive element is disposed in a tilted manner from perpendicularity relative to the optical axis of the objective lens so as to prevent flare from regular reflection.
In this case, a center of the diffractive element to be positioned with high accuracy relative to the optical axis of the objective lens corresponds to a center of one of the surfaces of the diffractive element facing the objective lens.
In order to position the center of the tilted surface with high accuracy as in the above-mentioned diffractive element, it is necessary to dispose a holding hole tilted relative to an objective lens holding hole in a casing. However, when such a shape is manufactured using a resin forming mold, it is impossible to manufacture the shape using a normal metal mold structure. A movable mold having a partial tilt is hardly capable of forming a shape with high accuracy for parts requiring reduction in size and weight as in this lens unit. In other words, such parts cannot be manufactured or even if the parts are manufactured, it is impossible to obtain the accuracy required for installation.
When members having such a complicated shape are used, there is an increased likelihood of occurrence of problems in basic functions and reliability.
Patent Document 1 discloses a structure in which the objective lens and the diffractive element are installed on the support member. However, by using such a dedicated support member, a size of the lens unit as a movable casing may be enlarged and the mass of a balancing weight for the lens unit may be increased. In accordance with this, sensitivity or a high resonance frequency is likely to be reduced due to the enlarged size and the increased mass.
It is an object of the present invention to solve the problems of the related art, and to provide a lens actuator, an optical pickup device, and an optical recording and reproducing apparatus that can be readily manufactured, perform positioning with high accuracy using an inertia weight, prevent heat transfer to a diffractive element, and improve reliability.